I would really encourage you to focus on the arcs in both your overall series and in your books. Like Storm said, each book has to have a complete beginning, middle, and end. You can have the series arc like in the Harry Potter books where each book leads up to the ultimate battle between Harry and Voldemort at the end. However, each book is a stand alone with its own set of problems and its own resolutions. There is the hanging question of what will happen with Voldemort left at the end of each book, but, for the most part, we are left feeling like the story presented is complete. The problems in the book are resolved (at least for the time being) and the characters have all grown in some way.

Maybe make a list of the things that happen in the series and then make a list of what happens in each book that brings the character through the series?

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